This invention relates to wavelength division mutiplexed (DWDM) systems used in optical fiber communications, and more particularly to optical signal filters which separate a WDM channel stream into groups of channels on separate fibers.
Narrow band optical filters are essential in wavelength division (WDM) communication systems in order to process signals at different precisely spaced wavelengths. Low insertion loss, flat top filter response, sharp cutoff, and the ability to scale to high channel counts and dense channel spacing are all critical parameters. An interleaving filter is a device or subsystem which can separate multiple channels in a WDM transmission into groups. A 1xc3x972 interleaving filter divides a WDM channel stream, periodically spaced in optical frequency, in a manner such that every other channel is, launched into one of two separate fibers. More generally a 1xc3x97N interleaving filter, separates every Nth channel into one of N fibers.
The interleaving function, more broadly speaking, includes establishing a periodic transmissivity characteristic within a given wider frequency band, so that there is virtually lossless transmission within incrementally spaced frequency channels, and in effect full signal rejection between the channels. Preferably, the transmissive pass bands are shaped with flat top response, so that laser wavelength shifts and other variations within the pass bands can be tolerated, thus reducing the stringency of performance specifications imposed on such active elements. Therefore, in multiplexing, channel spacings can be reduced with improved performance, while in demultiplexing closely spaced channels can be separated without requiring prohibitively precise individual components, such as add/drop filters. In demultiplexing, interleaving filters can also serve to reduce the component counts and serial insertion losses, because they separate signals in parallel fashion and can be cascaded to divide channels into a number of smaller groups before wavelength selective devices are used to add or drop individual wavelength signals.
The most common approach to interleaving filter design is based upon using unbalanced Mach-Zender interferometers. These are adequately responsive but are large, costly units that are difficult to adapt to many system requirements. In addition, they are subject to inherent instability problems that require extra measures to overcome. Thin film 200 GHz filters are now being offered, but thin films require costly and precise processes. Other periodic optical transmission functions are known, such as those exhibited by birefringent crystals, as delineated in detail by Yeh and Yariv in xe2x80x9cOptical Waves in Crystalsxe2x80x9d, John Wiley and Sons (1983). As the authors explain, a birefringent element sandwiched between parallel polarizers has a transmission characteristic that is periodic in optical frequency, and effectively without loss at transmissive peaks. Much analytical work, of both theoretical and practical natures has been directed to using the properties of birefringent crystals. In 1964, for example, Harris et al proposed a procedure for the synthesis of optical networks in an article in the Journal of the Optical Society of America, Vol. 54, No. 10 (October 1964), pp. 1267-1279, entitled xe2x80x9cOptical Network Synthesis Using Birefringent Crystalsxe2x80x9d. This article treats some of the considerations fundamental to synthesizing specific transfer functions using a series of birefringent crystals between entry and exit polarizers. Subsequently, Kimura et al discussed a technique for reducing thermally induced variations in an article entitled xe2x80x9cTemperature Compensation of Birefringent Optical Filtersxe2x80x9d, in the Proceedings of the IEEE, August 1971, pp. 1271-2. They disclosed that if the signs of the birefringence of two different crystals are opposite, the retardation of the series is less dependent on temperature. Although the intended purpose of the device described is as a filter for frequency stabilization, one of the articles cited, xe2x80x9cWide-band Optical Communication Systems, Part Ixe2x80x94Time Division Multiplexingxe2x80x9d, by T. S. Kinsel, Proc. IEEE, Vol. 58, October 1970, pp. 1666-1683 is referenced in regard to the use of birefringent optical filters to multiplex or demultiplex carriers of different frequencies in the field of wide-band optical communications.
A usage of crystals that is somewhat more related to the interleaving filter context is disclosed in a letter published in Electronics Letters, Vol. 23, No. 3 dated 29 January 1987, at pp. 106 and 107, by W. J. Carlsen et al, discussing the use of a series of birefringent crystals configured to improve the characteristics of systems disclosed by articles on prior tunable multiplexers/demultiplexers (referenced therein). All of these multiplexers are intended to be used with either of two lasers about 15 to 25 nm apart in optical wavelength, but they do not suggest features suitable for an interleaving function or operation at the now common 100 to 200 GHz spacings. A 100 GHz interleaving filter, for example, requires a passband of the order of 0.2 nm (vs about 10 for the Carlsen et al system) and like intermediate stop bands. Carlsen et al do discuss a modification which achieves a, flattened passband using five retardation plates of selected orientations relative to the end polarizers, and achieving polarization independence by splitting the beam so as to direct polarization components separately through the filter.
A need thus exists for a wideband interleaving filter having multiple narrow channel spacings and functioning with wide and flattened passband characteristics, insensitivity to polarization, temperature stabilization and very low insertion loss. The need includes a configuration made of readily available materials that can be readily assembled with the necessary precision, and that is of compact size and also mechanically stable.
Interleaving circuits for optical networks in accordance with the invention utilize a series of birefringent crystals in varying electrooptic property combinations and orientations to provide densely packed periodic transmission peaks which nonetheless have very low insertion loss, polarization independence, flattened passband peaks and temperature compensation. Pairs of dissimilar birefringent elements in cascaded (series) relationship broaden the transmissive peaks while compensating out the effects of temperature variations. By mounting the elements on a planar reference structure having preset recesses in which adjustments can be made, the unit can be aligned and adjusted with respect to retardation, spacings and orientation for best performance.
In a more specific example of an interleaving filter in accordance with the invention, birefringent crystals are arranged in series between an input and output beam displacing polarizers, together with beam combining elements at the output. The input beam is divided into two beams of orthogonal polarization, which are successively incident on two stages of paired birefringent crystals, the crystals of each pair being of opposite sign of thermooptic coefficient and of specific length ratios, and the crystals of the second pair being twice the length of the first. With crystals of yttrium orthovandate (YVO4) and lithium niobate (LiNbO3), respectfully, the ratio used is 6.60 to 1, and the crystals are precisely spaced apart and provided with anti-reflection coatings on the beam-incident surfaces. The lengths used are inversely related to the desired channel spacing. The optical (c) axes of the crystals are angled relative to the polarize d input signals and to each other to utilize the retardation difference of the birefringent crystals, providing two temperature compensated output beams having flatband maxima, which are then split into another set in an output beam splitting polarizer. One combined beam of both polarization components is collimated for direction to one output fiber, while two separate beams of orthogonal polarization are combined in a group of prisms and a polarizing beam splitter cube, for direction through a collimator to a second output fiber.
This interleaving filter, enclosed in a sealed housing, is less than 20 cm long and 5 cm wide. Placement and angular orientation of the optical elements is facilitated by the shaping of receiving recesses in the optical bench, along the optical beam path. Where a pyroelectric crystal such as LiNbO3 is used, buildup of surface changes due to temperature cycling is avoided by current conduction from the crystal faces that do not receive the optical beams. To achieve precise tuning, crystal faces may be angled so that relative translation of beams with respect to the crystals changes the path length within crystals.
In accordance with other features of the invention, beam displacing polarizers are used in recombination of beams which have passed through the birefringent crystal system. Path length differences can be equalized by employing a half wave plate in both beams or a compensating plate in one of the beams incident on the beam displacing polarizer.